The chapter introduces the concepts of uncertainty, risk, and reliability in geotechnical engineering, emphasizing the unique challenges and approaches compared to structural and mechanical engineering. It highlights the historical development of reliability methods in civil engineering, from the early work of pioneers like Alfred Freudenthal to the more recent applications in geotechnical fields. The chapter also discusses the philosophical and terminological issues surrounding probability and statistics, distinguishing between frequentist and degree-of-belief approaches to uncertainty. It explains the importance of recognizing the difference between aleatory (underlying randomness) and epistemic (lack of knowledge) uncertainties, which are crucial for effective risk and reliability analysis in geotechnical projects. Examples such as offshore platforms and pit mine slopes illustrate the practical application of these concepts, emphasizing the need for probabilistic and statistical methods to manage uncertainties in geotechnical design and construction.The chapter introduces the concepts of uncertainty, risk, and reliability in geotechnical engineering, emphasizing the unique challenges and approaches compared to structural and mechanical engineering. It highlights the historical development of reliability methods in civil engineering, from the early work of pioneers like Alfred Freudenthal to the more recent applications in geotechnical fields. The chapter also discusses the philosophical and terminological issues surrounding probability and statistics, distinguishing between frequentist and degree-of-belief approaches to uncertainty. It explains the importance of recognizing the difference between aleatory (underlying randomness) and epistemic (lack of knowledge) uncertainties, which are crucial for effective risk and reliability analysis in geotechnical projects. Examples such as offshore platforms and pit mine slopes illustrate the practical application of these concepts, emphasizing the need for probabilistic and statistical methods to manage uncertainties in geotechnical design and construction.